1. Field of the Invention
The present invention relates to a linear scale measuring device, and more particularly to a device employing cancellation of a moire error, and to a position detection method employing this measuring device.
2. Description of Related Art
A linear scale measuring device typically comprises a scale and a linear encoder. The scale is fixed to a mechanical part or device, for example, while the linear encoder is mounted to a movable portion of the device, such that a distance from a reference position or the like is measured.
For such a linear scale type measuring device to perform accurate measurement, the linear encoder must be appropriately positioned relative to the scale. A linear encoder which is inappropriately inclined with regard to the scale produces error, especially moire error.
For detecting an absolute position using such a measuring device, on the other hand, more than one scales must be simultaneously employed, because, with only one scale, it is not possible to specify which integral multiple of corresponds to the lattice pitch formed on the scale the detected position.
FIG. 3 is a conceptual structural view of a linear scale type measuring device for use in absolute position detection. Referring to FIG. 3, a plurality of scales 10a and 10b are disposed in parallel, and relative to these a linear encoder 12 is disposed. The linear encoder 12 includes therein a sensor 1 for detecting a position relative to the scale 10a and a sensor 2 for detecting a position relative to the scale lob.
Each of the sensors 1 and 2 comprises four detecting elements which are arranged at intervals of xcex/4 with regard to a wavelength xcex of each scale. Specifically, assuming that wavelengths of the scales 10a and 10b are set to xcex1 and xcex2, respectively, the four detecting elements of each sensor 1 and 2 are disposed at intervals of xcexxc2xc and xcex{fraction (2/4)}, respectively. When the linear encoder 1 moves, a detection signal is output from each detecting element of the sensor 1. Because the detecting elements are each provided with a displacement of xcexxc2xc from the adjacent element, the phases of the detection signals are also shifted by xcexxc2xc. Assume that the phase of the detecting element at the left end (first element) is a reference phase, for example, the phase of the detecting element (second element) adjacent to the first element is shifted by 90xc2x0, the phase of the detecting element (third element) adjacent to the second element is shifted by 180xc2x0, and the phase of the detecting element at the right end (fourth element) is shifted by 270xc2x0, respectively, from the reference phase. Accordingly, provided that detecting signals from the first and third elements are detecting signals in the x direction while detecting signals from the second and fourth elements are detecting signals in the y direction, a phase difference is detected by the following expression.
xcex81=arc tan(y/x)xe2x80x83xe2x80x83(1) 
The phase difference is then converted into a distance and a position relative to the scale 10a can be obtained by the following expression.
X1=xcex81/2xcfx80xc2x7xcex1xe2x80x83xe2x80x83(2) 
Similarly, the position relative to the scale 10b can be obtained with the sensor 2 by the following expression.
X2=xcex82/2xcfx80xc2x7xcex2xe2x80x83xe2x80x83(3) 
Since the reference points for the scales 10a and 10b coincide with each other, the absolute position with regard to such a reference point can be detected based on the above X1 and X2.
As described above, detection of an absolute position is possible when the linear encoder 12 is precisely positioned relative to the scales 10a and 10b. However, when the linear encoder 12 rotates about the center C and is inclined relative to the scales 10a and 10b as shown in FIG. 4, the detection values X1 and X2 from the sensors 1 and 2 are no longer accurate and this causes an error, specifically a moire error, to be included in the absolute position detected from X1 and X2.
In order to prevent such a moire error, a technique has been proposed in which a third scale which is the same as either of the scale 10a or 10b is further disposed in a symmetrical manner with regard to the center C, such that the detected values for the two same scales are averaged to cancel the moire error. This technique, however, requires 2nxe2x88x921 scales for n scales and therefore the size of an entire device increases, making downsizing difficult.
It is also possible to provide a moire angle sensor within the encoder for detecting angle data, which is then used to correct the detection values using a software such as a microcomputer. This technique, which requires a separate sensor, also suffers from the problem that it makes downsizing difficult.
The present invention has been made in view of the foregoing problems of the related art, and aims to provide a measuring device and a position detection method capable of accurate measurement with a simple structure while preventing a moire error.
To this end, a linear scale type measuring device according to one aspect of the invention comprises a scale, and a linear encoder which is disposed relative to said scale includes a plurality of sensors with regard to said scale, the plurality of sensors being collinearly arranged such that the array of sensors is inclined at a predetermined angle with respect to the longitudinal direction of said scale. The linear scale type measuring device of the present invention further comprises operation means for calculating a detection phase of said linear encoder based on phases detected by said plurality of sensors and said predetermined angle. When two sensors are employed, and providing that phases detected by the two sensors are "PHgr"A, and "PHgr"B, respectively, the initial phases of the two sensors are "PHgr"A0, and "PHgr"B0, respectively, said predetermined angle is xcex8, a wavelength of said scale is xcex, and a distance between the rotation center of the sensors and the center point between the two sensors when the sensors are inclined by the predetermined angle xcex8 is L, the operation means may calculate the detection phase of said linear encoder, in other words, the detection phase equal to that when the two sensors are not inclined with respect to the scale, using the following expression.
"PHgr"=("PHgr"A+"PHgr"B)/2xe2x88x922xcfx80/xcexxc2x7L{sin xcex8+("PHgr"A0xe2x88x92"PHgr"B0xe2x88x92"PHgr"A+"PHgr"B)/4xcfx80 sin xcex8xc2x7cos xcex8}xe2x80x83xe2x80x83(4) 
The detection phase can be converted into a distance, such that the position of the linear encoder with respect to the reference position of the scale can be obtained.
In order to detect the phase of each of the plurality of sensors, each of the plurality of sensors may, for example, comprise a plurality of sensor elements disposed at predetermined intervals, e.g. xc2xc of the scale wavelength. Any number of sensor elements may be provided.
A linear scale type measuring device according to another aspect of the present invention comprises a pair of scales; a linear encoder disposed relative to said pair of scales; and an operation unit for detecting the position of said linear encoder relative to said pair of scales based on an output from said linear encoder, wherein said linear encoder includes a plurality of sensors with regard to each of said pair of scales, said plurality of sensors being previously arranged such that they are inclined at a known angle with respect to the measuring direction of said scales, and wherein said operation unit detects the position of said linear encoder resulting from the inclination of said plurality of sensors with regard to the measuring direction of the scales, based on detection signals from said plurality of sensors and said known angle which is previously set. The position of the encoder resulting from the inclination of the sensors is accurately detected for each of said pair of scales, such that the absolute position of the linear encoder from the reference position can be obtained.
According to still another aspect of the present invention, there is provided a method of detecting a position using a linear scale type measuring device comprising a scale and a linear encoder. The method comprises the steps of (a) inclining a plurality of sensors arranged collinearly within said linear encoder at a predetermined angle with respect to said scale; (b) detecting each phase of said plurality of sensors using respective of said plurality of sensors; and (c) calculating a detection phase of said linear encoder based on the detected phases, said predetermined angle, and initial phases of said plurality of sensors. The detection phase may be obtained using the above expression.